1. Field of the Invention
The present invention relates to a method and an apparatus for hydroforming a metallic tube.
2. Description of the Related Art
Metallic tube hydroforming comprises the steps of introducing a hydraulic fluid into a metallic tube serving as a material tube (hereinafter, referred to merely as a metallic tube) and applying an axial force to the tube ends, to thereby form the metallic tube through combined use of hydraulic pressure and the axial force. The hydroforming process provides tubular parts having a variety of cross-sectional profiles.
FIGS. 7(a1), 7(a2), 7(b1), 7(b2), 7(c1), and 7(c2) show a metallic tube and products. FIG. 7(a1) is a side view showing a metallic tube, and FIG. 7(a2) is a front view showing the metallic tube. FIGS. 7(b1) and 7(c1) are side views of products obtained through tube hydroforming, and FIGS. 7(b2) and 7(c2) are front views of the products.
Each of the products includes an expanded portion 2a (3a) having a rectangular cross section and end portions 2b (3b) having the same outer diameter as a diameter D.sub.0 of a metallic tube 1. FIGS. 7(b1) and 7(b2) show a product 2 in which side lengths D.sub.1 and D.sub.2 of the expanded portion 2a are larger than the tube diameter D.sub.0.
FIGS. 7(c1) and 7(c2) show a product 3 in which at least one (in this case, D.sub.1) of side lengths D.sub.1 and D.sub.2 of the expanded portion 3a is smaller than the tube diameter D.sub.0. Overall lengths L.sub.1 and L.sub.2 of the products 2 and 3, respectively, are shorter than the length L.sub.0. of tube 1
First will be described a conventional hydroforming apparatus used for obtaining-the product 2.
FIGS. 8(a) and 8(b) show a die portion of the conventional hydroforming apparatus. FIG. 8(a) is a longitudinal sectional view showing the die portion. FIG. 8(b) is a sectional view taken along the line C--C of FIG. 8(a).
The die is composed of a lower die 4 and an upper die 5. The lower die 4 is attached to a bolster 10 of an unillustrated press unit. The bolster 10 is located at a lower portion of the press unit. The upper die 5 is attached to a ram head 11 of the press unit. The ram head 11 is located at an upper portion of the press unit. The ram head 11 is moved vertically by means of an unillustrated hydraulic cylinder so as to press the upper die 5 against the lower die 4 with a predetermined force. Die cavities 4a, 5a and a tube-holding groove 4b,5b for containing a metallic tube therein are formed in the upper and lower die 4,5. When the upper and lower dies 5 and 4 are closed each other, a space defined by the die cavities 4a and 5a is used for forming the expanded portion 2a of a product. The contour of the die cavities is identical to the external contour of the expanded portion 2a of a product. When the upper and lower dies 5 and 4 are closed each other, a space defined by the die cavities 4a and 5a is used for forming the expanded portion 2a of a product. The contour of the die cavities is identical to the external contour of the expanded portion 2a of a product. When the upper and lower dies 5 and 4 are closed each other, the diameter of the space defined by the tube-holding grooves 4b and 5b is identical to the outer diameter D.sub.0 of the metallic tube 1. Left- and right-hand sealing-punch 6 and 7 are attached to unillustrated corresponding horizontal press units. The left- and right-hand sealing-punch 6 and 7 advance toward or retreat from the left- and right-hand tube-holding grooves 4b and 5b, respectively.
Next will be described a hydroforming process for obtaining the product 2 through use of the above-mentioned conventional hydroforming apparatus.
FIGS. 9(a1), 9(a2), 9(b1), 9(b2), 9(c), and 9(d) illustrate a conventional hydroforming process. FIG. 9(a1) is a longitudinal sectional view showing a metallic tube set in the upper and lower dies. FIG. 9(a2) is a sectional view taken along the line C--C of FIG. 9(a1). FIG. 9(b1) is a longitudinal sectional view showing a final state of hydroforming. FIG. 9(b2) is a sectional view taken along the line C--C of FIG. 9(b1). FIG. 9(c) is an enlarged view showing the encircled portion a of FIG. 9(b2). FIG. 9(d) is a perspective view showing a product ruptured during hydroforming.
As shown in FIGS. 9(a1) and 9(a2), first, the metallic tube 1 is set in the tube-holding grooves 4b formed in both end portions of the lower die 4. The ram head 11 is lowered so as to press the upper die 5 against the lower die 4. The sealing punches 6 and 7 are advanced from their respective sides so that head portions 6a and 7a of the sealing punches 6 and 7, respectively, are tightly inserted into both end portions of the metallic tube 1, thereby the tube ends are sealed during hydroforming. Next, while a hydraulic fluid 8 is introduced into the metallic tube 1 by means of an unillustrated pump through a path 6b extending through the left-hand sealing punch 6, air inside the metallic tube 1 is ejected through a path 7b extending through the right-hand sealing punch 7. An unillustrated valve located on the extension of the path 7b is closed after the interior of the metallic tube 1 is filled with the hydraulic fluid 8.
An example of the hydraulic fluid 8 is an emulsion prepared by dispersing a fat-and-oil component in water in an amount of several percent so as to produce a rust-preventive effect. The pressure of the hydraulic fluid 8 contained in the metallic tube 1 is increased with advancing the sealing-punch 6 and 7 to press the metallic tube axially. Thus, the material of the metallic tube 1 is expanded within the die cavities 4a and 5a to form the product 2 as shown in FIGS. 9(b1) and 9(b2).
The upper and lower dies 5 and 4 are pressed against each other during the hydroforming in order to prevent the upper die 5 from being pressed upward off the lower die 4 when the metallic tube 1 is expanded through the application of fluid pressure and axial force. Axial pressing is performed in order to feed the material of the metallic tube 1 located in the tube-holding grooves 4b and 5b into the die cavities 4a and 5a, to thereby minimize the wall thinning of an expanded portion of the product 2.
Subsequently, the internal fluid pressure of the product 2 is reduced to atmospheric pressure. Then, the upper die 5 is moved upward, and the sealing punches 6 and 7 are retreated, thereby draining the hydraulic fluid 8 from inside the product 2. The product 2 is ejected from the lower die 4.
Next will be described a conventional hydroforming process for obtaining the product 3. FIGS. 10(a1), 10(a2), 10(b1), and 10(b2) illustrate conventional dies used for obtaining the product 3 through hydroforming. FIG. 10(a1) is a longitudinal sectional view of a set of lower die 14 and upper die 15. FIG. 10(a2) is a sectional view taken along the line C--C of FIG. 10(a1). FIG. 10(b1) is a longitudinal sectional view of an another set of lower die 24 and upper die 25. FIG. 10(b2) is a sectional view taken along the line C--C of FIG. 10(b1).
In FIGS. 10(a1) and 10(a2), the rectangular cross section of a space defined by die cavities 14a and 15a of a lower die 14 and an upper die 15, respectively, is profiled such that a vertical side length D.sub.1 is shorter than a horizontal side length D.sub.2. In FIGS. 10(b1) and 10(b2), the rectangular cross section of a space defined by die cavities 24a and 25a of a lower die 24 and an upper die 25, respectively, is profiled such that a horizontal side length D.sub.1 is shorter than a vertical side length D.sub.2.
In hydroforming with either the die shown in FIG. 10(a1) or the die shown in FIG. 10(b1), a round metallic tube can not be used, as will be described later.
In the case of the die shown in FIG. 10(a1), the round tube is set on the die cavity 14a of the lower die 14, not on the tube holding groove 14b. When the upper die 15 is lowered, the tube will be crushed between the die cavities 14a and 15a.
FIGS. 11(a) and 11(b) are sectional views showing deformed states of the metallic tube crushed between the lower die 14 and the upper die 15. FIG. 11(a) shows a deformed state of the metallic tube within the die cavities, and FIG. 11(b) shows a deformed state of the metallic tube within the tube-holding grooves.
As shown in FIG. 11(a), when the upper die 15 is lowered while a metallic tube 16 is set in the die cavity, the tube 16 is deformed within the die cavity into a cocoon shape with side-wall bucklings 17. This also causes generation of bucklings 18 on portions of the tube 16 within the tube-holding grooves near the die cavities.
When these bucklings are clamped between the upper and lower dies 15 and 14, a product and the dies 15 and 14 must be damaged.
In order to avoid the occurrence of the bucklings, the round metallic tube must be preformed into a shape which can be inserted within the die cavities and the tube holding grooves.
Also, in the case of the die shown in FIG. 10(b1), a round metallic tube must be preformed; otherwise, the die cavities 24a and 25a cannot contain the metallic tube.
FIGS. 12(a1), 12(a2), 12(b1), and 12(b2) are views illustrating the above-mentioned preforming process. FIG. 12(a1) is a longitudinal sectional view showing a state in which a round metallic tube 1 is set in a flattening die 30 while plugs 32 are inserted into both ends of the tube. FIG. 12(a2) is a sectional view taken along the line C--C of FIG. 12(a1). FIG. 12(b1) is a longitudinal sectional view showing a state in which a punch 31 is lowered from above with an unillustrated press unit to thereby flatten the round metallic tube 1. FIG. 12(b2) is a sectional view taken along the line C--C of FIG. 12(b1).
As shown in FIG. 12(a1), a die cavity width D.sub.2 ' of the die 30 is made slightly smaller than the width D.sub.2 of the die cavities 14a and 15a shown in FIGS. 10(a2) and 10(b2). The plugs 32 are used for prevent deformation of the tube ends which will be held in the tube-holding grooves 14b and 15b of the dies 14 and 15, respectively. A plug head portion 32a has substantially the same diameter as an inside diameter of the tube. The plug 32 is positioned by contacting a flange 32b to a tube end.
As shown in FIG. 12(b1), a punch 31 is lowered from above with an unillustrated press unit so as to flatten the metallic tube 1 to a height D.sub.1 ', yielding a locally flattened tube 33. The height D.sub.1 is made slightly smaller than the die cavity width D.sub.1 shown in FIGS. l0(a2) and (b2). The cross section of a flattened portion 33a of the flattened tube 33 becomes a cocoon shape. However, die walls 30a prevent the occurrence of the backings 17 as shown in FIG. 11(a). The plugs 32 also prevent generation of the bucklings 18 as shown in FIG. 11((b).
The flattened metallic tube 33 is set in the dies 14 and 15 of FIG. 10(a1) or in the dies 24 and 25 of FIG. 10(b1) and undergoes hydroforming.
FIGS. 13(a1), 13(a2), 13(b1), and 13(b2) are sectional views illustrating a tube hydroforming process conducted through use of the dies 14 and 15 of FIG. 10(a1). FIG. 13(a1) is a longitudinal sectional view showing the flattened metallic tube 33 set in the dies 14 and 15. FIG. 13(a2) is a sectional view taken along the line C--C of FIG. 13(a1). FIG. 13(b1) is a longitudinal sectional view showing a state after the completion of hydroforming the flattened metallic tube 33. FIG. 13(b2) is a sectional view taken along the line C--C of FIG. 13(b1). As shown in FIG. 13(a1), the flattened metallic tube 33 is set in the die cavity 14a and in the tube-holding grooves 14b of the lower die 14. The upper die 15 is lowered and pressed against the lower die 14 with a predetermined force, and the sealing punches 6 and 7 are advanced from their respective sides so as to insert the punch head portions 6a and 7a into the end portions of the flattened metallic tube 33, thereby sealing the punches 6 and 7 against corresponding tube ends. The flattened metallic tube 33 is filled with the hydraulic fluid 8. The pressure of the hydraulic fluid 8 is gradually increased so as to expand the flattened portion 33a having a cocoon-shaped cross section within the die cavities 14a and 15a, yielding a product formed along the die profile as shown in FIGS. 13(b1) and 13(b2).
Two problems are involved in the conventional hydroforming process for obtaining the product 2 or the like described previously with reference to FIGS. 9(a1), 9(a2), 9(b1), 9(b2), 9(c), and 9(d).
A first problem is wall thinning which occurs at four corner portions of a cross section of the expanded portion 2a as encircled in FIG. 9(b2). As the ratio of a circumferential length S2 of the expanded portion 2a of a product 2 to a circumferential length S0 of a metallic tube, S2/S0, increases or as a radius r of a corner portion as shown in the enlarged view of FIG. 9(c) decreases, the degree of wall thinning of a corner portion increases. Accordingly, a product may fail to n obtain required wall thickness, or excessive wall thinning may cause a rupture 70 at a corner portion as shown in FIG. 9(d). At a required corner radius smaller than a critical value, the conventional hydroforming process may be inapplicable especially to a tube material having a relatively high strength, since the ductility of such material is poor.
Through feed of a tube material in tube-holding grooves into a die cavity by axial pressing with the sealing punches 6 and 7, wall thinning at corner portions can be suppressed to some degree. However, when a length L of the expanded portion 2a of a product is relatively long, the effect of axial pressing does not reach an axially central section of the expanded portion 2a. Thus, a wall thinning problem at corner portions still exists.
According to an experiment conducted by the inventors of the present invention when, for example, a carbon steel tube having a 40 kgf/mm.sup.2 -class tensile strength is hydroformed into a product whose expanded portion 2a has a length L four times a tube diameter D.sub.0 and a square cross section with S2/S0=1.25 (S2: circumferential length of the expanded portion 2a; S0: circumferential length of the tube), the corner radius r cannot be made less than or equal to 5 times a wall thickness t (see FIG. 9(c)).
The degree of wall thinning at a corner portion is larger than that at a flat side portion. This is because during hydroforming expansion an increase in the diameter of a metallic tube is maximized in a diagonal corner-to-corner direction. Flat side portions of a product come into contact with the walls of the die cavities 14a and 15a at a relatively early stage of hydroforming. Thus, the extensional deformation of the flat side portions in a circumferential direction is suppressed by the friction between the flat side portions and the die cavity walls. This promotes the extensional deformation of corner portions in a circumferential direction.
A second problem is that in hydroforming there must be a relatively high pressure of the hydraulic fluid 8. In the conventional hydroforming process as described previously with reference to FIGS. 9(a1), 9(a2), 9(b1), 9(b2), 9(c) and 9(d), an internal pressure p must be applied to a metallic tube in order to form a corner portion with a radius r as shown in FIG. 9(c). The required internal pressure p can be estimated by the following equation. EQU p=(tx.sigma.)/r
where t is the wall thickness of a tube material, and .sigma. is the strength of a tube material.
For example, with t=3 mm, .sigma.=50 kgf/mm.sup.2, and r=15 mm, p is calculated as 10 kgf/mm.sup.2, i.e., a high pressure of 1,000 atm is required for hydroforming. As the pressure of the hydraulic fluid 8 increases, a pressure generator becomes further large-scaled, and a larger force is required for pressing upper and lower dies each other. Accordingly, since die strength must be increased, a hydroforming apparatus becomes expensive, resulting in an increase in hydroforming cost.
Also, two problems are involved in the conventional hydroforming process for obtaining the product 3 or the like described previously with reference to FIGS. 13(a1), 13(a2), 13(b1), and 13(b2)
A first problem is the wall thinning of the expanded portion 3a of the product 3; particularly, wall thinning which occurs at corner portions of a cross section of the expanded portion 3a. In hydroforming as illustrated in FIGS. 13(a1), 13(a2), 13(b1), and 13(b2), resistance which arises when a tube material passes through stepped portions 14c and 15c of the dies 14 and 15, respectively, hinders smooth pushing of the tube material in the tube-holding grooves 14b and 15b into the die cavities 14a and 15a. As a result, the degree of wall thinning at corner portions becomes rather large even when a length L of the expanded portion 3a is relatively short.
A second problem is a shape defect of a rectangular sectional profile as shown in FIG. 13(b2). This problem derives from a metallic tube to be hydroformed with a cocoon shape as shown in FIG. 13(a2).
FIGS. 14(a) to 14(c) illustrate generation of the shape defect. FIG. 14(a) is a sectional view showing an initial stage of hydroforming. FIG. 14(b) is a sectional view showing an intermediate stage of hydroforming. FIG. 14(c) is a sectional view showing a final stage of hydroforming.
As shown in FIG. 14(a), in an initial stage of hydroforming, the pressure of the hydraulic fluid 8 causes convex portions 35 of the cocoon shape to come into contact with the walls of the die cavities 14a and 15a. Subsequently, as the fluid pressure increases, the depth of concave portions 34 decreases gradually. As shown in FIG. 14(b), area of the zones 36 in contact with the die cavity walls gradually increases with the increase of the fluid pressure. Due to friction of between the contact zones 36 and the die cavity walls, the concave portions 34 are no longer deformed. While a tube material of corner portions 37 is extending in a circumferential direction, a corner radius r gradually becomes smaller. Since a circumferential material length of the concave portion 34 is excessive, the concave portions 34 cannot be brought into contact with the die cavity walls even when the fluid pressure is increased. As a result, as shown in FIG. 14(c), the concave portions 34 remain in a product. The above-mentioned problems are also involved in hydroforming through use of the die shown in FIG. 10(b).